Electric power conversion systems are used to condition the electric power supplied to motor load circuits from a DC source of relatively constant voltage. If supplying DC motors, such a system will include an electric power "chopper" that is suitably controlled to vary the magnitude of load current and/or voltage as desired. Alternatively, in the case of alternating current (AC) motors, the system will include an electric power "inverter" that is suitably controlled to vary the amplitude and frequency of load voltage as desired. In either case, electric power flows from the DC source terminals to the load terminals of the controllable converter during "motoring" operation or in a reverse direction during "electrical braking".
Such a system is useful for propelling a rapid transit vehicle, in which case the source comprises a wayside conductor and the load comprises windings of at least one traction motor whose rotatable shaft is mechanically coupled through torque-increasing gearing to an axle-wheel set of the vehicle. The wayside conductor is typically energized by a relatively low voltage DC power generating plant located near the right of way along which the vehicle travels. In its motoring or propulsion mode of operation, the converter is so controlled that the DC voltage applied to its source terminals is converted into adjustable voltage at its load terminals, and the traction motor(s) responds by producing torque to accelerate the vehicle or maintain its speed as desired.
In the alternative electrical braking or retarding mode of operation of the power conversion system, the converter is so controlled that each motor acts as a generator driven by the inertia of the vehicle and supplies electric power which flows in a reverse direction through the converter and appears as direct and unipolarity voltage at the source terminals. As this electrical energy is used or dissipated, the traction motor(s) responds by absorbing kinetic energy and slowing the vehicle. Electrical braking is achieved by a combination of dynamic braking and regenerative braking. Dynamic braking is effected by connecting a dynamic braking resistance between the DC source terminals. This resistance receives current from the converter, converts the electrical energy to thermal energy, and dissipates the resulting heat. Regenerative braking, on the other hand, is effected by returning to the DC power source power flowing in a reverse direction through the converter during braking operation. These two electrical braking modes can be combined in desired proportions, this mixing process being commonly referred to as "blending".
A power conversion system including a voltage source inverter for supplying AC traction motors is disclosed in U.S. Pat. No. 3,890,551--Plunkett, assigned to General Electric Company. An important feature of the Plunkett power conversion system is its inclusion of ohmic resistance (shown at 28 in FIG. 1 of the Plunkett patent) that is inserted into the DC link between the inverter and the DC power source during electrical braking but is effectively removed from the DC link during motoring. By inserting this series resistor during electrical braking, the magnitude of voltage at the DC terminals of the inverter can increase above that of the source voltage. One of the advantages of raising the inverter voltage is to enable the traction motors to develop more magnetic flux for braking and to use less current than would otherwise be required for very high braking effort.
The power conversion system of the Plunkett patent also includes a low pass electrical filter of the conventional series inductance (L), shunt capacitance (C) type between the voltage raising resistor and the inverter for attenuating harmonics generated by operation of the inverter and for partially isolating the inverter from undesirable line transients. (As used herein, the term "harmonics" refers to various components of the composite current and voltage waveforms having frequencies that are multiples of the frequency of the fundamental component of such waveforms.) In addition, the shunt capacitance of the filter at the DC terminals of the inverter provides the "stiff" voltage required for proper operation of a voltage source inverter.
The desired blending of dynamic and regenerative braking can be accomplished in various different ways that are well known to persons skilled in the art. See, for example, U.S. Pat. No. 4,093,900--Plunkett. In the present state-of-the-art, it is preferable to replace the parallel array of separate braking resistors and their respectively associated electromechanical switches, as shown in U.S. Pat. No. 4,093,900, with a single bank of resistance elements connected to the DC link via an electric power chopper comprising a controllable solid-state electric valve that can be repetitively turned on and off in a pulse width modulation (PWM) mode to control the average magnitude of current in the resistor as desired. An example of this modern practice is disclosed in U.S. Pat. No. 4,761,600--D'Atre et al., where the electric valve comprises a main thyristor for commutating the main SCR from a conducting state (on) to a non-conducting or current blocking state (off). Alternatively, a solid-state gate turn-off device (GTO) could be substituted for the chopper shown in U.S. Pat. No. 4,761,600.
The filter capacitors used to provide the filtered DC link voltage in the above described systems are generally electrolytic capacitors and have a higher failure rate than many other power components. Typically, the filter capacitors may range from 10000 to 100,000 microforads (MFD) and are formed from a plurality of parallel connected capacitors. For example, as many as 112 individual capacitors may be used to create a single 55,000 MFD capacitance means. One of the primary functions of these capacitors, in addition to "smoothing" the DC link voltage is to reduce certain frequencies of current which can be introduced to the wayside conductors DC power source from the propulsion system. As is well known, such wayside conductors are often positioned adjacent wayside signalling equipment in transit applications. The signalling equipment may operate at preselected frequencies, such as, for example, 25 Hz, 60 Hz, 95 Hz, 200 Hz, or such other frequency as the transit authority may select. The signalling system may be used for communication to transit vehicles operating in the system or to indicate the presence of a transit vehicle within a particular block of the transit system. Other frequencies, such as 360 Hz, 720 Hz, and 990 Hz, are used for safety checks as is explained in co-pending U.S. patent application Ser. No. 07/630,698, filed Dec. 20, 1990, and assigned to the assignee of the present invention, the disclosure of which is hereby incorporated by reference. Because of the importance of the signals on the signalling system, it is desirable that transit vehicles not generate signals in their respective propulsion systems which might interfere with the signalling system. To this end, the values of the capacitance means and the inductance means in the power filter circuit are selected to avoid oscillations or ringing at signalling frequencies or harmonics of these frequencies. However, as noted above, the electrolytic capacitors used in the filter circuits are known to have higher failure rates than other components. Accordingly, it is desirable to provide a method for periodically verifying the value of the capacitance means so that capacitors whose value has changed may be replaced. Such maintenance not only assures integrity of the filter circuit but can be used to direct maintenance personnel to the capacitors in case of degradations and assures smoother operation of the propulsion system with adequate capacitance means.
As discussed above, the capacitance means also operates in conjunction with the electrical braking system for the transit vehicle. A more detailed description of the operation of an electrical braking system may be had by reference to U.S. Pat. No. 4,904,918--Bailey et al., issued Feb. 27, 1990 and assigned to the assignee of the present invention. During electrical braking of the transit vehicle, the capacitance means is called upon to attenuate harmonics generated by the operation of the chopper in varying the dynamic braking resistance.